...her mom says that her shoulder is rubbing the plastic piece where you thread the car's seat belt.
I think the first question to ask is whether the headrest of the Frontier is in the correct position. If the belt positioning guides are rubbing against the child’s shoulders it could need moving up one notch/click. I have no experience of using the Frontier however many here on the forum do and may be able to offer better advice.
If the seat is simply outgrown, then a special needs seat is probably the only solution if the child needs to remain harnessed (the Frontier has high harness slots versus many other seats). While there are many possible options, seats classified as special needs devices tend to be very expensive although insurance may cover the costs.
If the Frontier does prove unsuitable, the first option is another harnessed seat. These include:
Merritt Manufacturing
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Roosevelt – Forward facing harnessed seat which caters for children from 35 to 115 pounds and up to 62” tall.
Snugseat
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Traveller Plus – Very similar to the former Britax Husky/Regent. Forward facing, harnesses to 105lbs.
Columbia Medical
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2000 Therapedic™ IPS Car Seat - Forward facing harnessed seat to 102lbs. Support and positioning accessories available.
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2500 Therapedic™ IPS Car Seat – Similar to above but has an upper weight limit of 130lbs. Again, support and positioning accessories available.
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2400 ‘Spirit’ APS Car Seat – Can be used for occupants between 25 and 130lbs and up to 66” tall.
The second option would be a booster seat, with an integrated or additional positioning harness for support:
Merritt Manufacturing
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Churchill – Booster for children weighing up 175lbs. Again, various support accessories available.
Snugseat
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Pilot – High back booster for children weighing up to 120lbs. I think this is based on the Sunshine kids Monterey booster, but I’m not completely sure.
Recaro Reha
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Monza Reha Adaptive Car Seat – Booster with torso support harness for users up to 110lbs.
The third option would be a vest:
E-Z-On Products
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E-Z-On Vests come in various models which are used frequently in special needs transport. We currently use an E-Z-On Vest and are very happy with it. They can be used for occupants up to 168lbs. We discovered it was possible to keep the vest secured to the seat, leaving the buckles open when it is not in use. This way when it is needed the person using it can sit down and simply put it on like a jacket. It also makes it easier to assist someone using the vest if they have trouble managing on their own.
There are plenty of experts here on the forum who are trained in special needs seating who can offer far better advice and knowledge but I hope this helps.